Nanomaterials

Nature is full of scales both in space and time where a variety of structures, that is, materials of different characteristics, find their home. These structures, either from nature or human-made, are not only physically and/or biologically interesting, but also technologically relevant if one can use them in different applications. Such a multitude of scales encompass the smallest at the quantum level, 10−11−10−8 (m) in length and 10−16 −10−12 (s) in time, next to the atom/nano level, 10−9−10−6 (m) in length and 10−15−10−10 (s) in time, mesoscale, 10−6−10−3 (m) in length and 10−10−10−6 (s) in time, and the macroscopic one, less than 10−3 (m) in length and less than 10−6 (s) in time.

In addition, in the domain of the continuum mechanics, the so-called microscopic (few microns) scale is also important for many engineering applications. In fact, the classic textbook Transport Phenomena by R. Byron Bird et al., has an excellent introduction to multiscale process by using three of the scale levels mentioned above: the molecular, the microscopic, and the macroscopic scales. The authors actually suggest that an understanding of the “bulk” behavior can be systematically obtained by an up-scaling or “bottom-up” approach based at the molecular level. Thus, “macroscopic transport equations” are suggested based on knowledge at the molecular and microscopic level. This upscaling or bottom-up approach is what is now frequently [Page 499]used to describe material processing in nanotechnology as opposed to the downscaling or top-down approach used in miniaturization.

The prefix nano, derived from Greek terminology, implies “dwarf” and has been used to describe structures, that is, materials and their associated characteristics, whose sizes are within less than 100 nm at least in one dimension. This type of system is the core of the nanotechnological applications. Now nanotechnology is a word frequently used to indicate the “bottom-up” approach mentioned above to manufacture “bulk” materials. Furthermore, nano is also frequently used to indicate that the resolution of the system is at the molecular level.

One very important parameter in nanomaterials is the surface/volume ratio of the system. This parameter undergoes a dramatic increase in value when the size of the system reaches the nanoscale level. At this stage, the importance of the surface acquires a different magnitude since about half of the atoms of the system are located on the surface domain.

Under this condition, the surface can display unique properties quite different to those of the bulk of the material. This is a key factor in contributing to the different behavior of the material at the nanoscale with respect to the one at the bulk scale. In general, this increase of the surface/volume ratio leads to an alteration of the physical (thermal, diffusion), chemical (catalytic activity), and electrical (conductivity) and optical properties.

For example, noble metals such as platinum and gold that at the bulk scale are inert become powerful catalytic agents when they reach the nanoscale; aluminum that is stable under usual conditions becomes combustible; copper that is ordinarily opaque transforms into a transparent material at the nanoscale; and insulators such as silicon become conductors. One key reason behind the behavior observed is because when the system shows an energy-surface dominated performance the quantum and statistical mechanics effects such as the “quantum size effect” becomes predominant under reduced-sized systems.

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